4_corrosion And Its Prevention

3 Day PIP on Metallurgy for Non Metallurgist from 10th to 12th April 2017

Corrosion and its Prevention

Mr. Rahul Gupta

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Corrosion

ASM International – Pune Chapter. Metallurgy for Non‐Metallurgists.

Corrosion.

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Corrosion What is Corrosion? • Corrosion has been described as destruction of metal by chemical or Corrosion from electrochemical reaction with its environment. This section describes the types and chemical processes of corrosion, and some of the Environments common methods used to prevent it. • Most environments are corrosive, but by no means to the same degree. • Environments that foster corrosion include: air and moisture; fresh, distilled, or saltwater. , rural, urban, and industrial atmospheres; steam and other gases such as chlorine, ammonia, hydrogen sulfide, sulfur‐dioxide, fuel gases; mineral acids such as hydrochloric, sulfuric, and nitric; organic acids such as naphthenic, acetic, and formic; alkalies; soils; solvents such alcohols and dry cleaning fluids; vegetable and petroleum oils; and a variety of food products. In general, the inorganic materials are more corrosive. Pune Chapter

Corrosion • For example, corrosion in the petroleum industry is due more to sodium chloride, sulfur, hydrochloric and sulfuric acids, water than the oil, naphtha, or gasoline. Corrosion of steel and iron by air and moisture, referred to as rusting, is very common and results in tremendous corrosion and loss. The rapid rusting of clean, unprotected iron and steel is well known to everyone. . • Corrosion rates vary widely for almost any given metal with geographical location. For instance, a specimen of polished carbon steel exposed to a rural inland atmosphere will rust at a very slow rate, due to the low humidity and no contamination from industrial fumes or saline atmospheres. Such a location would represent about the most favorable environmental condition for minimizing corrosion.

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Corrosion • A subsequent step in increasing corrosivity might be in a coastal area where a certain amount of saline atmosphere would be involved. Under these conditions, corrosion rates will further increase— probably several fold over the rural, low‐humidity conditions. • A coastal location near a chemical plant where humidity is high and quantities of industrial fumes are present. Under these conditions, corrosion rates of most common metals (without specific protection) are extremely high. Probably the near‐ultimate conditions for maximum • Corrosivity of a specific metal or metal alloy will vary greatly in conventional atmospheric exposure, depending upon geographical location and atmospheric conditions. Thus, selection of metals and/or the means of protecting them is a dynamic problem in the metals industry. Our examples have illustrated just one of the many factors that influence atmospheric corrosion.

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Corrosion • There are many other factors adding to the complexity of the problem of estimating corrosion rates and selecting materials that will provide acceptable performance. Some of the questions to consider are: • Will the metal be used to contain or handle corrosive materials, such as food items or chemicals? • Will the metal be used in an ambient temperature, or at elevated or cryogenic (low) temperatures? • What types of loads will be applied to the component in service? • Effect of Temperature:‐ Almost without exception, whether the environment is mild or aggressive, corrosion rate for an otherwise constant set of conditions will increase as temperature increases. Further, this is seldom a straight‐line relationship. The corrosion rates increase exponentially as temperature increases.

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Corrosion • General Corrosion can be classified as: Wet or Dry; direct combination or electro chemical; or by corrosive media including atmospheric (gas corrosion), • Classification liquid (aqueous or non‐aqueous), and soil (underground). • Wet corrosion occurs when liquids are present and at temperatures below the dew point. • Dry corrosion occurs in the absence of liquids or above the dew point. Vapors and gases are usually the corroding environment. Dry corrosion is generally associated with high temperatures. An example would be the corrosion of steel by furnace gases. Dry corrosion at ordinary temperatures is usually very slow. • Sometimes the presence of moisture or water changes the corrosiveness of the environment completely. For example, dry chlorine at room temperature is practically non corrosive to ordinary steel. Wet chlorine, however, is extremely corrosive and attacks most of the common metals and alloys. The reverse is true for Titanium, which corrodes in dry chlorine gas.

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Corrosion • Direct Corrosion. Direct combination, or direct corrosion, usually occurs at high temperature, similar to dry corrosion. It involves a reaction between a metal and nonmetallic elements or compounds, such as steam, oxygen, sulfur, and chlorine. Using the example of iron in an atmosphere containing sulfur dioxide (dry corrosion), the reactions are: Fe + SO = FeS + O 2Fe +02= 2FeO • Electrochemical corrosion occurs in liquids or electrolytes (solutions containing ions). Most liquids, such as plain water, seawater, acids, and other chemicals, are good conductors of electricity. Most of the corrosion is caused by liquids.

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Corrosion • Electrochemical Theory. This is the accepted theory explaining corrosion, and it is actually quite simple. For an electrochemical reaction to take place, there are two basic requirements: (a) anodes and cathodes must be present to form a cell, and (b) direct current must flow. The anodes and cathodes may be very close together (local cells), or they may be far apart. The current may be self‐ induced, or it may be impressed on the system from an outside source. • The anode is the area where corrosion occurs and where current leaves the metal and enters the solution. The cathode is the area where no corrosion occurs and where current enters the metal from the solution. Anodes and cathodes can form on a single piece of metal because of local differences in the metal or in the environment. • The metal at the anode dissolves and becomes an ion. It is oxidized and loses electrons. . These electrons are accepted at the cathode area. Pune Chapter

Corrosion • Immersion in an electrolyte or conducting fluid is required to complete the circuit to carry the current (electrons) from the anode or anodic area to the cathode or cathodic area. For example, high (purity) water is used in certain applications to keep corrosion at a very low rate. However, most waters such as tap water and seawater are not pure and are good conductors. • Figure 3. Hydrogen ions accept electrons at the cathode and form hydrogen gas. This is visible & is called polarization of the cathode. Polarization of a local cathode by a layer of hydrogen minimizes corrosion. Fe (metal) + 2H (ions) = Fe (ion) + H (gas) • The quantity of current that passes through this cell is proportional to the amount of metal that corrodes; that is, one ampere per year equals approximate 10kg of steel. Pune Chapter

• Figure 3

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Corrosion • Anything that interferes with these reactions or the cell circuit could/ reduce corrosion. In pure water, hydrogen bubbles collect on the cathode, thus providing an insulating blanket that reduces current flow and practically stops corrosion. This is called polarization; because it occurs on the cathode, it is termed cathodic polarization (see Figure 4), In strong acid solutions, the hydrogen is continuously evolved as bubbles breaking from corroding steel with the corrosion continuing unabated until either all the metal or acid is consumed. • Most waters contain dissolved oxygen. This oxygen combines with the hydrogen to form water and thus prevents accumulation of hydrogen (polarization), and corrosion proceeds unhampered. Oxygen acts as a cathodic depolarizer in this case. ‘This is the reason why boiler water is de aerated (see Figure 5).

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Figure 4 & 5

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Corrosion • In neutral or alkaline waters (containing dissolved oxygen) the anode reaction involving the corrosion of iron is the same, but the cathode reaction is as follows: H + ½0 + 2e = 20H (hydroxyl ion) leading to the formation of Fe(OH)2 • Corrosion of steel by water containing oxygen. When depolarization occurs (hydrogen and oxygen combine to form water) corrosion again proceeds. • The anodic and cathodic areas must be separated by a finite distance. These areas could also be far apart a meter (yard) or more in some cases. For example, one piece of pipe might act as the anode and the section next to it as a cathode.

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Corrosion • The reason the same piece of metal acts differently is that differences exist at the interface on the metal surface itself. The differences may be chemical or metallurgical in nature, depending on the character of the metal. Examples are impurities, such as oxides and other inclusions; grain boundaries; orientation of grains; differences in composition of the microstructure; localized stresses; and scratches and nicks or other rough surfaces. • Figure 6 summarizes the points just made and may be considered a basic diagram illustrating corrosion of metals in electrolytes or aqueous solutions. Anodes and cathodes must form,, current must flow through the environment (outside the metal) from the anodes to cathodes, and through the metal from the cathodic to the anodic areas. These are the basic requirements as stated above. Preventing any of the above will prevent corrosion. Pune Chapter

Figure 6

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Corrosion • When two dissimilar metals are in contact, for example copper and steel, one usually behaves as the anode , while the other as cathode. The metal with less corrosion resistance becomes the anode (or anodic) and the more corrosion‐resistant metal becomes the cathode (or cathodic). For the example given, iron becomes the anode and copper becomes the cathode. Accordingly, it is said that steel is anodic to copper. Dissimilar metals in contact form galvanic cells. Any anode and cathode form a galvanic cell, but to avoid confusion the term galvanic corrosion shall be reserved for corrosion caused by dissimilar metals. • When an anode and cathode form a potential difference, or voltage, exists between the anode and cathode. Accurate determinations of potentials for various elements resulted in the emf series shown in Table I.

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Table 1

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Corrosion • In this instance, hydrogen electrode is the reference electrode. The potential of hydrogen is listed as zero, If platinum were used as a reference electrode, the voltage for platinum would be listed as zero. This table is determined under a standard set of conditions including temperature and concentration of ions in the solutions. • This series ranks the elements according to their relative tendency to corrode. • Metals high in the series are reactive and less corrosion resistant. • Metals lower in the series are more noble or more corrosion resistant. • For example, metals above copper corrode or oxidize readily.

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Corrosion • This is one reason why gold and silver are often found in their native state, whereas iron and aluminum are always found in the combined state and usually mined as oxides. • A metal will replace another metal in solution if the latter metal is lower in the series. For example, iron will replace copper when placed in a copper sulfate solution (copper “plates out’). In other Words, copper sulphate will corrode iron. • A metal higher in the series will act as the anode, and one lower will act as a cathode when these two metals are coupled or are in contact with each other. • When two metals are in contact, corrosion of the anodic member of the couple is accelerated. This is called galvanic corrosion, or two‐ metal corrosion.

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Corrosion • Passivity in a metal refers to a relatively inactive state in which the metal displays a more noble behavior than thermodynamic considerations predict. Passivity can be more simply defined as the reason why a metal does not corrode when it should. According to the emf series, chromium is above iron, and, therefore, chromium is more reactive or less corrosion resistant than iron (or plain carbon steel, which is usually over 99% iron). If chromium is added to iron to make a high chromium (over 12%) steel, this alloy should be less corrosion resistant than plain carbon steel. On the contrary, these alloys are the stainless steels .Chromium and the stainless steels become passive, or exhibit passivity, because of the formation of a protective film on the surface of the metal.

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Corrosion • An oxide or an absorbed oxygen film acts as a barrier, thus protecting the metal from the environment. • Aluminum is an active metal and is high in the emf series, but it possesses‐good corrosion resistance. This metal quickly forms a surface coating of aluminum oxide, which stops further corrosion by many environments. Titanium also forms .a protective film of Ti0 and high silicon cast iron forms a protective silica coating. • Lead is above hydrogen in the emf series, yet it shows excellent resistance to sulfuric acid at concentrations below 90%. Lead sulfate quickly forms when lead is exposed to sulfuric acid and corrosion stops. • Passivity is a relative term. Steel dipped in nitric acid is passive to copper sulfate, but it would not resist 10% sulfuric acid. A stainless steel may exhibit passive behavior in nitric acid and in water, but it would be attacked by strong hydrochloric acid. Any metal or alloy is passive or active only in relation to some particular environments. ‐

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Corrosion • Some of the corrosion classifications are unique and distinct, but in most instances they are, in some way, interrelated. The listing that follows is some what arbitrary, it covers most forms of corrosion and failures caused by corrosion. • Nine common forms are listed as follows: 1. Uniform attack or general overall corrosion 2. Galvanic or two‐metal corrosion 3. Concentration‐cell corrosion 4. Pitting corrosion 5. Selective leaching 6. Inter granular corrosion 7. Stress‐corrosion cracking 8. Erosion‐corrosion 9. Crevice corrosion

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Corrosion • Uniform Corrosion Corrosion of metals by uniform chemical attack is the simplest and most common form of corrosion and occurs in the atmosphere, in liquids, and in soil, frequently under normal service conditions. The rate of attack can be rapid or slow, and the metal surface can either be clean or be covered with corrosion products. • Selection of a metal that has a suitable resistance to the environment in • which the specific part is used and the application of paints and other types of coatings are two common methods used to control uniform corrosion. • Uniform corrosion commonly occurs on metal surfaces having a homogenity of chemical composition and of microstructure .All metals are affected by this form of attack in some environments: the rusting of steel and the tarnishing of silver are typical examples of uniform corrosion. In some metals, such as steel, uniform corrosion produces a somewhat rough surface by removing a substantial amount of metal, which either dissolves in the environment or reacts with it to produce a loosely adherent, porous coating of corrosion products. In reactions such as the tarnishing of silver in air or the attack on lead in sulfate‐ containing environments, thin, tightly adherent protective films are produced, and the metal surface remains smooth. Pune Chapter

Corrosion • Corrosion rate and expected service life can be calculated from measurements of the general thinning produced by uniform corrosion. However, since the rate of attack can change over a period of time, periodic inspection at suitable intervals is ordinarily done to avoid unexpected failures. • Modification of the environment by changing its composition, concentration, pH and temperature, or by adding an inhibitor, are also effective and appropriate methods of controlling uniform corrosion in some situations. • Effect of Concentration. The effect on corrosion rate of increasing or decreasing the concentration of corrodent in the environment to which a metal is exposed does not follow a uniform pattern—first, because of ionization effects in aqueous solutions and the effects of even trace amounts of water in nonaqueous environments, and second, because of changes that occurr in characteristics of any film of corrosion products that may be present on the surface of the metal. Pune Chapter

Corrosion • Figure 9. Effect of acid concentration on the corrosion rate of iron completely immersed in aqueous solutions of three inorganicacids at room temperature; (a)hydrochloric acid (b) sulfuric acid, and(c) nitric acid. It should be noted that the scales for corrosion rate are not the same for all three charts. The corrosion rate of iron (and steel) in nitric acid in concentrations of 70% or higher, although low compared to the maximum rate, is still sufficient to make it unsafe to ship or store nitric acid in these metals. • The rate of corrosion of a given metal usually increases as the concentra tion of the corrodent increases, as shown in Figure 9(a) for the corrosion of iron in hydrochloric acid. However, corrosion rate does not always increase with concentration of the corrodent; the effect often depends on the range of corrodent concentration as shown in Figure 9(b) and (c) for iron in sulfuric acid and in nitric acid, respectively. Nitric acid in bulk is usually stored and shipped in type 304 stainless steel, aluminum alloy 3003, or commercially pure titanium (grade 2).

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Corrosion Figure 9.

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Corrosion • Metals that have passivity effects corrode at an extremely low rate at low acid concentration at room temperature, but lose their passivity at a certain limiting acid concentration above which the corrosion rate increases rapidly with increasing acid concentration. • Effect of Temperature. In investigating the effect of temperature on the corrosion rate of a metal in a liquid or gaseous environment, the temperature that must be considered is that existing at the metal/corrodent interface, which often differs substantially from the temperature of the main body of the corrodent. • This is especially important for heat‐transfer applications where‘hot‐ wall” effect causes a much higher corrosion rate . A rise of 10°C ( temperature of the solution can increase the Corrosion rate by a factor of 2 or more. Hot‐wall failures are fairly common in heat exchanger tubes. • In some metal‐corrodent systems. there an exponential rise in corrosion rate with an increase in temperature Pune Chapter

Corrosion • The steel water system is an exception. As the temperature is raised, the oxygen content of the water reduces, especially as the boiling point of the water is approached. • Other exceptions arise where a morderate increase in tempériture results in the formation of a thin protective film on the surface of the metal or in passivation of the metal surface.This is important in tempered steel for temperatures upto 450oC. The Fe3O4 formed actually protects the steel. • If thick deposits are formed on a heat‐transfer surface, they have a two fold effect by (a) changing the metal surface temperature and (b) making crevice corrosion possible. • When dissimilar metals are in electrical contact in an electrolyte, the less Galvanic Corrosion metal (anode) is attacked to a greater degree than if it were exposed alone, and the more noble metal (cathode) is attacked to a lesser degree than if it were exposed alone.

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Corrosion • This behavior which is known as galvanic corrosion, can often be recognized by the fact that the corrosion is more severe near the junction of the two metals than elsewhere on the metal surfaces. Galvanic corrosion is usually the result of poor design and selection of materials or the plating‐out of a‐more noble metal from solution on a less noble metal. • The greater the difference in potential between the two metals, the more rapid will be the galvanic attack. The electromotive force series ranks the metals according to their chemical reactivity, but applies only to the laboratory conditions under which the reactivity was determined. In practice, the solution potential of metals is affected by the presence of passive or other protective films on some metals, polarization effects, degree of aeration, and temperatures.Refer to the galvanic corrosion series for commonly used engineering materials. Materials in the same category can be safely paired.

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Corrosion • If it is impossible to avoid the use of dissimilar metals in cementing materials, the following precautions should be observed. • The electrical conductivity of the cementing material should be kept to a minimum. • The use of other ionic compounds should be forbidden. • Contamination of the cementing material, by use of sea water , should be avoided. • Abnormally delayed drying and subsequent exposure to moisture should also be prevented as far as possible.

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Corrosion • Dissimilar metals should be spaced as far apart as practicable. • Coating the anode should be avoided, unless the coating is continuous or unless the intense localized corrosion that may occur at a few uncoated points can be tolerated. If one of the two dissimilar metals is to be coated, coat the cathode. • Concentration‐cell corrosion occurs on buried metals as a result of their being in contact with soils that have different chemical compositions, water contents, or degrees of aeration. • Pitting of materials is extremely localized corrosion that generally produces Pitting Corrosion sharply defined holes. Every enginecring metal or alloy is susceptible to pitting. Pitting occurs when one area of a metal surface becomes anodic in respect to the rest of the surface, or when highly localized changes in the corrodent in contact with the metal, as in crevices, cause accelerated localized attack. • Severe pitting corrosion at the water line of ships can be observed. This is due to salt concentration variatons due to level changes. Pune Chapter

Corrosion • Pitting on clean surfaces ordinarily represents the start of breakdown of passivity or local breakdown of inhibitor‐ produced protection. • When pits are few and widely separated, and the metal surface undergoes little or no general corrosion, there is a high ratio of cathode‐ to‐anode area and penetration progresses more rapidly than when pits are numerous and close together. • Difficulty of Detection. Pitting is one of the most difficult to detect forms corrosion; it can cause failure by perforation while producing only a small weight loss on the metal.

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Corrosion • Some causes of pitting are local inhomogeneity on the metal surface, local loss of passivity. mechanical or chemical rupture of a protective oxide , galvanic corrosion from a relatively distant cathode, and the formation of a metal iron or oxygen concentration cell under a solid deposit (crevice corrosion).. • The most common causes of pitting in steels are surface deposits that set up local concentration cells and dissolved halides that produce local anodes by rupture of the protective oxide film. Anodic corrosion inhibitors, such as chromates, can cause rapid pitting if present in concentrations below a minimum value that depends on the metal‐ environment combination, temperature, and other factors. • Pitting occurs at mechanical ruptures in protective organic coatings if the external environment is aggressive, or if a galvanic cell is active. • In corrosion‐resistant alloys, such as stainless steels, the most common cause of pitting corrosion is highly localized destruction of passivity by contact with moisture that contains halide ions and in particular, chlorides.

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Corrosion • Pitting of Specific Metals. Despite their good resistance to general corrosion, stainless steels are most susceptible to pitting than many other metals. The stainless steels higher in chromium, nickel, and molybdenum are also higher in resistance to pitting, but are not immune under all service conditions. • Pitting failures of corrosion‐resistant alloys are relatively uncommon in solutions that do not contain halides • Pitting of aluminum and magnesium and their alloys in aqueous solutions is often accelerated by galvanic effects, which occur because these metals are anodic to most other metals, and by the effects of dissolved metallic ions and suspended particles in the solution.

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Corrosion • Selective Leaching Selective leaching is the removal of an element from an alloy by corrosion. The most common example is dezincification. that is, the selective removal of zinc in brasses. Many alloys are susceptible to selective leaching under certain condition. The elements that are more resistant to the environment remain behind, provided that they have a sufficiently continuous structure to prevent them from breaking away in small particles. • Dezincification occurs in brasses containing less than 85% copper. Zinc corrodes preferentially, leaving a porous residue of copper and corrosion products. Alpha brass containing 70% copper and 30% zinc is particularly susceptible to dezincification when exposed in an aqueous electrolyte at elevated temperatures.

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Corrosion • Austenitic stainless steels become sensitized or susceptible to inter‐ granular corrosion when heated in the temperature range of about 550 to 850 °C(1020 to 1560°F).These temperatures are encountered during manufacturing processess such as welding. • In the sensitizing range, chromium carbides and carbon precipitate out of solution if the carbon content is about 0.02% or higher. The chromium carbide in the grain boundary is not attacked, but in many corrosive environments the chromium‐depleted zone that is immediately adjacent to the grain boundair is attacked. • One method of reducing the susceptibility of austenitic stainless steels to inter granular corrosion is to use solution heat treatment, usually by heating to 1066 to 1121°C (1950th 2050°F) and immediately water quenching. By this procedure, chromium carbide is dissolved and retained in solid solution,. • Solution heat treatment poses problems on many welded assemblies and is generally impractical on large equipment

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Corrosion • Using stainless steels that contain less than 0.03% carbon (extra‐low‐ carbon grades) reduces susceptibility to inter granular corrosion sufficiently for serviceability in many applications. Somewhat better performance can be obtained from types 347 or 321 stainless steel, which contain sufficient titanium and niobium (or niobium plus tantalum), respectively, to combine with all of the carbon in the steel. • The latter approach is used to correct the condition illustrated and described in Figure 16.

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Corrosion

Figure 16

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Corrosion • Stress‐corrosion cracking is a mechanical‐enviromnental failure process In combine to initiate and cracking a metal part. Stress‐corrosion cracking is produced by the synergistic action of sustained tensile stress and a specific corrosive environment, causing failure in less time than would the separate effects of the stress and the corrosive environment if applied together. • Failure by stress‐corrosion cracking frequently is caused by simultaneous exposure to a seemingly mild chemical environment and to a tensile stress well below the yield strength of the metal. Under such conditions, fine cracks can penetrate deeply into the pail while the surface exhibits only faint signs of corrosion. Hence, there may be no macroscopic indications of an impending failure. • In addition to stress‐corrosion cracking, them are several other processes that cause failure of metal parts by the conjoint action of mechanical stress and corrosion. These include hydrogen damage (hydrogen embrittlement), corrosion fatigue, liquid metal embrittlement, and fretting. • Stress‐Corrosion sustained tensile stress and chemical attack

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Corrosion • Sources of Stresses in Fabrication. The principal sources of high local stresses in manufacture include (a) thermal processing, (b) stress raisers, (c) surface finishing. and (d) fabrication. • Thermal Processing. One of the most frequently encountered sources of thermal‐processing stresses is welding. Shrinkage of weld metal during cooling, and fixtures, can produce residual tensile stresses as high as 207 to 276 MPa (30 to 40 ks,) Other thermal‐processing effects that often produce stresses during manufacture include solidification of castings (especially those having large differences in section thickness and those made with cast‐in inserts) and improper heat treating practices (failure to preheat, when required; overheating during solution treating; failure to provide required temperature uniformity in furnaces; use of quenching practices too severe for a specific alloy or part shape; and undue delay in transferring work pieces from the quenchant to the tempering furnace).

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Corrosion • Surface Finishing. Residual tensile stresses that are damaging or potentially damaging in conjunction with environmental attack are produced in many different types of surface‐finishing treatments. These treatments . include electroplating, electrical discharge machining, and (under some conditions) conventional grinding and machining. When hydrogen is produced in the finishing process and diffuses into the metal, internal stresses can be produced. • Shot peening and surface rolling often are used to produce compressive stresses in metal surfaces, but the magnitude of stress imposed by peening is sometimes not great enough to overcome the effects of extremely high local tensile stresses. • Fabrication. High residual tensile stresses sometimes result from bending, stamping, deep drawing, and other cold‐forming operations. Residual tensile stresses of 207 to 414 MPa (30 to 60 ksi) have been measured on the surfaces of cold bent steel tubes.

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Corrosion • Under some circumstances, severe, uniform cold working improves the resistance of a metal to stress‐corrosion cracking. For example, cold‐ drawn steel wire is more resistant to stress‐corrosion cracking than is oil‐tempered wire having equal mechanical properties. Also, cold reduction of low‐carbon steel to 50% or less of its original thickness makes it relatively immune to cracking in boiling nitrate solutions at 100 to 200°C (212 to 392 °F) for thousands of hours. • Assembly. Fit‐up and assembly operations often are sources of tensile stresses. Press fitting, shrink fitting, and assembly by welding are among the major operations in this category. • Forming operations used in assembly to retain components can produce residual tensile stresses that can induce stress‐corrosion cracking, particularly when the parts are used or stored in a corrosive atmosphere. • When movement of a corrodent over a metal surface increases the rate of ‐ Erosion‐Corrosion attack due to mechanical wear and corrosion, the attack is called erosion‐ corrosion. All flowing or turbulent corrosive media can cause erosion‐corrosion.

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Corrosion • Impingement corrosion is a severe form of erosion‐corrosion. • Steam erosion is another form of impingement corrosion. It occurs when high‐velocity wet steam contacts a metal surface. • Crevice Corrosion A crevice in a metal surface, at a joint between two metallic surfaces or between a metallic and a nonmetallic surface, or beneath a particle of solid matter on a metallic surface, provides conditions conducive to the development of the type of concentration‐ cell corrosion called crevice corrosion. • In a metal‐ion concentration cell, the accelerated corrosion occurs at the edge of or slightly outside of a crevice. .

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Corrosion • Effect of Solid Deposits. In power‐generation equipment, crevice corrosion failures have occurred in main‐station condenser tubes cooled with seawater, as a result of the formation of solid deposits and the attachment of marine organisms to the tube wall; these failures have occurred particularly in condensers tubed with stainless steel. • Riveted and bolted Joints must be considered as possible sites for crevice corrosion, and thus they require careful attention in design and assembly to avoid crevices, as well as provisions to ensure uniform aeration and moderate but not excessive flow rates at the joints. . • • Where a corrosion failure has occurred, economical and practical measures for prevention of future failures of the same type are required.

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Corrosion Types of corrective and preventive measures are: • Change in alloy, heat treatment, or product form • Use of resinous and inorganic‐base coatings • Use of inert lubricants • Use of electrolytic and chemical coatings and surface  treatments . • Use of metallic coatings • Use of galvanic protection • ‐ Design changes for corrosion control • Use of inhibitors • Changes in pH and applied potential • Continuous monitoring of variables

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Corrosion •

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Extreme temperatures and high operating stresses favor the use of special‐purpose cast alloys. For example, the tubes in gas reformers are usually, made of cast heat‐resisting alloys to obtain maximum life at operating temperatures of 871 to 1010°C . Similarly, the use of high‐alloy castings enables turbine engines to operate at higher temperatures (and, therefore, at higher efficiencies) than weldments would permit. Solution annealing of austenitic stainless steels minimizes the risk of intergranular attack and stress‐corrosion cracking. For stainless steel weldments that cannot be annealed and that are to be used in applications where either intergranular corrosion or intergranular stress‐ corrosion cracking is of concern ,a quenching procedure can be used. In this procedure, the weld is rapidly quenched to prevent the precipitation of the chromium carbides. if the quenching procedure proves ineffective, either a low‐carbon or a stabilized stainless steel may be substituted, depending on the tensile requirements. Acrylics, epoxies, phenolics, furanes, and urethanes are used extensively for Resinous and Inorganic‐ corrosion protection in the form of paints, potting compounds, adhesives, Base Coatings coatings, and linings. Their chemical resistance makes them suitable for many applications.

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Corrosion • However, especially for fairly corrosive immersion service, consideration must always be given to the possible presence of pinhole porosity in paint‐type coatings, which may result in pitting and crevice corrosion, and to the possibility of accelerated galvanic attack where dissimilar metals are present. • Sealant materials, if properly applied, are highly effective in preventing crevice corrosion; if not properly applied. they can make it more likely to occur. • Zinc‐Rich Coatings. Organic and inorganic coatings containing zinc dust give excellent protection to steel and galvanized structures. They provide sacrificial protection because the zinc particles are in intimate contact with one another, so that the coating fihu is electrically conductive. • Inert Lubricants Certain chemically inert resins (such as silicones, esters, and fluorocarbons) can serve both as effective lubricants and as corrosion‐resistant coatingsand linings. In seacoast environments, lubricants frequently must perform this dual role of lubrication and corrosion protection. The role of corrosion protection is often overlooked when selecting a lubricant for a specific function, as for wire rope on exposed sliding surfaces.

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Corrosion • The major types of eletrolytic and chemical coatings and surface treatments ,include electroplating,anodizing, chemical conversion coatings, and passivation treat‐ Chemical Coatings and rnents. These processes vary widely in their effectiveness . Anodic and chemical conversion coatings also serve as excellent bases for organic coatings. • Anodizing of aluminum and aluminum alloys provides effective protection in natural environments, but not against aggressive environments, especially acidic and alkaline environments. • Chemical Conversion Coating. Chromate conversion coatings are extensively used on steel products that have been electroplated with cadmium or zinc, and they substantially improve the corrosion resistance. Similar coatings are also produced by electrolytic processing, with the parts being made the anode in the electrolytic chrornisting bath. Chromate coatings however, are very thin and can readily be removed by abrasion or impact. Local bare areas then may corrode preferentially. • Chromate, phosphate, and other conversion coatings are also used on aluminum, magnesium, steel, and other metals, but primarily as a base to improve the adhesion and protective value of organic coatings to be applied over them.

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Corrosion • Passivation is common practice in the manufacture of stainless steel components and assemblies. . • Electro deposition of zinc or cadmium is widely used to protect steel from Metallic Coatings corrosion. Zinc provides better performance in industrial areas; cadmium is preferred for marine environments. These coatings offer sacrificial protection to the steel substrate • Nickel, chromium, and copper are other metals that are readily applied by electroplating but coatings of these metals are much less effective than sacrificial coatings in providing corrosion resistance. • Sprayed Metal Coatings. Metal spraying can provide thick protective coatings. Multiple coatings minimize, but do not completely eliminate, the occurrence of voids and weak spots in the coatings. Zinc and aluminum coatings are commonly used. • Cladding can provide thick coatings that arc free of even fine porosity and, with suitable selection of the cladding metal, give more effective protection against corrosion of the base metal. Cladding is costly, however, and cannot be applied to parts of all configurations.

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Corrosion • Galvanic Protection Galvanic protection can be either cathodic protection (in which the object to be protected is made cathodic) or anodic protection (in which the object to be protected is made anodic). The method most commonly used is • Cathodic protection. • Cathodic protection may be of two different types: impressed direct current or sacrificial anode. • In the impressed direct current type, the structure to be protected is made the cathode in a direct‐current electrical circuit. The anode in the circuit is an auxiliary electrode, usually of iron or graphite, located some distance away from the structure to be protected. The positive terminal of the source of direct current is connected to the auxiliaiy electrode, and the negative terminal to the structure to be protected. Current then flows from the electrode through the electrolyte to the stmcture, and the structure does not corrode. The applied voltage need only be high enough to supply an adequate current density to all parts of the structure to be protected.

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Corrosion • In the sacrificial anode system, the structure to be protected is made the cathode in a galvanic‐corrosion cell, and current is supplied by the corrosion of anodes that are commonly made of zinc or magnesium. Voltage and current are limited by the corrosion of the sacrificial anodes; the number and location of anodes are much more critical in this method • Anodic Protection. By imposing an external potential to make them anodic, some metals can be prevented from corroding in an electrolyte in which they would otherwise be attacked. This technique, which is called anodic protection, is applicable only to metals and alloys that show active‐passive behavior. It has been applied to iron, titanium, aluminum, and chromium, but mostly to steel and stainless steel. Anodic protection is not applicable to zinc, magnesium, cadmium, silver, copper, and copper‐base alloys.

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Corrosion • Anodic versus Cathodic Protection. Anodic and cathodic protection tend to ‘complement one another; each method has its advantages and disadvantages. Anodic protection can be used in corrosives ranging from weak to very aggressive; cathodic protection is restricted to moderately corrosive conditions because of its high current requirement, which increases as the corrosivity of the environment increases. • Inhibitors Anodic inhibitors stifle the anodic reaction, usually forming sparingly soluble substances as adherent protective films. Salts such as hydroxides, silicates, borates. phosphates, carbonates, andbenzoates are effective on steel only in the presence of dissolved oxygen. • Cathodic inhibitors stifle the cathodic reaction, either by restricting the access of oxygen or by “poisoning” sites favorable for cathodic hydrogen evolution. Cathodic inhibitors that decrease the corrosive action of aqueous solutions on steel include salts of magnesium, manganese, zinc, and nickel. l

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Corrosion • •

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Changes in pH and Applied Potential Pourbaix diagrams indicate the conditions for which diffusion bather films may form on an electrode surface, but they provide no measure of how effective r films may be in the presence of specific anions . A Pourbaix diagram for iron in water and dilute aqueous solutions is shown in Figure 20. Note that there are zones of corrosion, immunity from corrosion, and passivity.

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Corrosion • Continuous monitoring of critical variables in both batch operations and Continuous Monitoring process streams is a valuable tool . Some of the variables monitored and their applications are noted and described below. • Electrical Resistance (ER). • Linear Polarization Resistance (LPR). • Electrical Conductivity. The corrosion rate of many metals in organic systems increases significantly if a corrodenc is present and the electrical conductivity increases above IO mho. • Continuous Chemical Analysis.

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Corrosion

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